For the burgers, place torn bread in a bowl, cover with milk and let soak. Place pork and beef in a bowl and season with salt and pepper. Coarsely chop mortadella or prosciutto, and place in a food processor. Pulse machine to process into a fine chop. Add the minced meat to the bowl with the pork and beef along with the egg. Wring excess milk out the bread and mash it into moist crumbs between your fingers as you add to the meat bowl.
Add the garlic, rosemary, parsley, onion, add a couple handfuls of grated cheese, and nutmeg. Mix the meats well with your hands to combine. Drizzle with EVOO.
Blanch spinach leaves in boiling water then cold shock them in a bowl of ice water. Let dry on kitchen towels. Layer a few leaves of spinach and wrap them around a hard-boiled egg half.
Heat a nonstick griddle pan or large, nonstick skillet over medium-high heat.
Take 1/4 of the meat and form a long, loaf-like burger patty around each spinach-wrapped egg. Cook burgers 6 minutes on each side under a tinfoil tent.
Toss the reserved tomatoes with basil and a little EVOO.
Serve burgers open-faced on bread, topped with tomatoes and basil, and a drizzle of balsamic as garnish.